This invention relates to positron emission tomography (PET) scanners, such as those used for medical imaging; and particularly to circuits employed in PET scanners to detect a positron emission event.
Positrons are positively charged electrons which are emitted by radionuclides that have been prepared using a cyclotron or other device. The radionuclides are employed as radioactive tracers called "radiopharmaceuticals" by incorporating them into substances, such as glucose or carbon dioxide. The radiopharmaceuticals are injected into the patient and become involved in such processes as blood flow, fatty acids, glucose metabolism, and protein synthesis.
Positrons are emitted as the radionuclides decay. The positrons travel a very short distance before encountering an electron, and when that occurs, the position is annihilated and converted into two photons, or gamma rays. This annihilation event is characterized by two features which are pertinent to PET scanners--each gamma ray has an energy of 511 keV and the two gamma rays are directed in nearly opposite directions. An image is created by determining the number of such annihilation events at each location within the scanner's field of view.
The PET scanner has a ring of detectors that encircle the patient. The detectors comprise crystals, referred to as scintillators, to convert the energy of each 511 keV photon into a flash of light that is sensed by a photomultiplier tube. Coincidence detection circuits connect to the detectors and record only those photons that are detected simultaneously by two detectors located on opposite sides of the patient. The number of such simultaneous events indicates the number of positron annihilation that occurred along a line joining the two opposing detectors. Within a few minutes hundreds of million of events are recorded to indicate the number of annihilation along lines joining pairs of detectors in the ring. These numbers are employed to reconstruct an image using well known computed tomography techniques.
Gamma rays are also produced by naturally occurring events which cause a signal pulse to be produced by the photomultiplier tube. A signal pulse also may result from noise in the photomultiplier tube and other electronic noise in the processing circuitry. As a consequence a mechanism has to be provided to qualify the photomultiplier tube signal to select gamma radiation from positron emission and prevent noise from being misinterpreted as a positron emission event.
Previous PET scanners employed a constant-fraction discriminator to reject electronic noise and generate a low jitter time mark when a positron emission event occurs. The time mark are used by the coincidence circuit to reject spurious radiation. With reference to FIG. 1, a constant-fraction discriminator works by comparing two signals derived from an input signal produced by the radiation detector. One of these signals is the input signal delayed and the other is the input signal attenuated in magnitude. A timing mark indicating a qualified radiation event was developed by determining when these two signals are equal, which is equivalent to determining when the difference of the two signals is zero. The delayed input signal was produced by a lumped inductor-capacitor type analog delay line which utilized a large number of discrete inductors and capacitors. This implementation had expense and size drawbacks because of the large number of constant-fraction discriminators required in a PET scanner, i.e. one for each radiation detector.